Primary (non-rechargeable) electrochemical cells having an anode of lithium are known and are in widespread commercial use. The anode is comprised essentially of lithium metal. Such cells typically have a cathode comprising manganese dioxide, and electrolyte comprising a lithium salt such as lithium trifluoromethane sulfonate (LiCF3SO3) dissolved in an organic solvent. The cells are referenced in the art as primary lithium cells (primary Li/MnO2 cells) and are generally not intended to be rechargeable. Alternatively, primary lithium cells with lithium metal anodes but having different cathodes are also known. Such cells, for example, have cathodes comprising iron disulfide (FeS2) and are designated Li/FeS2 cells. The iron disulfide (FeS2) is also known as pyrite. The Li/MnO2 cells or Li/FeS2 cells are typically in the form of cylindrical cells, typically AA size or AAA size cells, but may be in other size cylindrical cells. The Li/MnO2 cells have a voltage of about 3.0 volts which is twice that of conventional Zn/MnO2 alkaline cells and also have higher energy density (watt-hrs per cm3 of cell volume) than that of alkaline cells. The Li/FeS2 cells have a voltage (fresh) of between about 1.2 and 1.8 volts which is about the same as a conventional Zn/MnO2 alkaline cell. However, the energy density (watt-hrs per cm3 of cell volume) of the Li/FeS2 cell is higher than a comparable size Zn/MnO2 alkaline cell. The theoretical specific capacity of lithium metal is high at 3861.4 mAmp-hr/gram and the theoretical specific capacity of FeS2 is 893.6 mAmp-hr/gram. The FeS2 theoretical capacity is based on a 4 electron transfer from 4Li per FeS2 molecule to result in reaction product of elemental iron Fe and 2Li2S. That is, 2 of the 4 electrons change the oxidation state of +2 for Fe+2 in FeS2 to 0 in elemental iron (Fe0) and the remaining 2 electrons change the oxidation state of sulfur from −1 in FeS2 to −2 in Li2S.
Overall the Li/FeS2 cell is much more powerful than the same size Zn/MnO2 alkaline cell. That is, for a given continuous current drain, particularly at higher current drain over 200 milliAmp, the voltage is flatter for longer periods for the Li/FeS2 cell than the Zn/MnO2 alkaline cell as may be evident in a voltage vs. time discharge profile. This results in a higher energy output obtainable from a Li/FeS2 cell compared to that obtainable for a same size alkaline cell. The higher energy output of the Li/FeS2 cell is more clearly and more directly shown in graphical plots of energy (Watt-hrs) versus continuous discharge at constant power (Watts) wherein fresh cells are discharged to completion at fixed continuous power outputs ranging from as little as 0.01 Watt to 5 Watt. (As the cell's voltage drops during discharge the load resistance is gradually decreased, raising the current drain to maintain a fixed constant power output.) The graphical plot Energy (Watt-Hrs) versus Power Output (Watt) for the Li/FeS2 cell is above that for the same size alkaline cell. This is despite that the starting voltage of both cells (fresh) is about the same, namely, between about 1.2 and 1.8 volt.
Thus, the Li/FeS2 cell has the advantage over same size alkaline cells, for example, AAA, AA, C or D size or any other size cell in that the Li/FeS2 cell may be used interchangeably with the conventional Zn/MnO2 alkaline cell and will have greater service life, particularly for higher power demands. Similarly the Li/FeS2 cell which is a primary (nonrechargeable) cell can also be used as a replacement for the same size rechargeable nickel metal hydride cell, which has about the same voltage (fresh) as the Li/FeS2 cell. Thus, the primary Li/FeS2 cell can be used to power digital cameras, which require operation at high pulsed power demands.
The cathode material for the Li/FeS2 cell may be initially prepared in a form such as a slurry mixture (cathode slurry), which can be readily coated onto the metal substrate by conventional coating methods. The electrolyte added to the cell must be a suitable organic electrolyte for the Li/FeS2 system allowing the necessary electrochemical reactions to occur efficiently over the range of high power output desired. The electrolyte must exhibit good ionic conductivity and also be sufficiently stable, that is non reactive, with the undischarged electrode materials (anode and cathode components) and also non-reactive with the discharge products. This is because undesirable oxidation/reduction side reactions between the electrolyte and electrode materials (either discharged or undischarged) could thereby gradually contaminate the electrolyte and reduce its effectiveness or result in excessive gassing. This in turn can result in a catastrophic cell failure. Thus, the electrolyte used in Li/FeS2 cell in addition to promoting the necessary electrochemical reactions, should also be stable to discharged and undischarged electrode materials. Additionally, the electrolyte should enable good ionic mobility and transport of the lithium ion (Li+) from anode to cathode so that it can engage in the necessary reduction reaction resulting in Li2S product in the cathode.
An electrode composite is formed with a sheet of lithium, a sheet of cathode composite containing the FeS2 active material and separator therebetween. The electrode composite may be spirally wound and inserted into the cell casing, for example, as shown in the spirally wound lithium cell of U.S. Pat. No. 4,707,421. A cathode coating mixture for the Li/FeS2 cell is described in U.S. Pat. No. 6,849,360. A portion of the anode sheet is typically electrically connected to the cell casing which forms the cell's negative terminal. The cell is closed with an end cap which is insulated from the casing. The cathode sheet can be electrically connected to the end cap which forms the cell's positive terminal. The casing is typically crimped over the peripheral edge of the end cap to seal the casing's open end. The cell may be fitted internally with a PTC (positive thermal coefficient) device or the like to shut down the cell in case the cell is exposed to abusive conditions such as short circuit discharge or overheating.
The electrolyte used in a primary Li/FeS2 cells is formed of a “lithium salt” dissolved in an “organic solvent”. Representative lithium salts which may be used in electrolytes for Li/FeS2 primary cells are referenced in related art, for example, as in U.S. Pat. No. 5,290,414 and include such salts as: Lithium trifluoromethanesulfonate, LiCF3SO3 (LiTFS); lithium bistrifluoromethylsulfonyl imide, Li(CF3SO2)2N (LiTFSI); lithium iodide, LiI; lithium bromide, LiBr; lithium tetrafluoroborate, LiBF4; lithium hexafluorophosphate, LiPF6; lithium hexafluoroarsenate, LiAsF6; Li(CF3SO2)3C, and various mixtures. In the art of Li/FeS2 electrochemistry lithium salts are not always interchangeable as specific salts work best with specific electrolyte solvent mixtures, and specific solvent mixtures with certain lithium salts can lead to significantly improved performance.
In U.S. Pat. No. 5,290,414 (Marple) is reported use of a beneficial electrolyte for FeS2 cells, wherein the electrolyte comprises a lithium salt dissolved in a solvent comprising 1,3-dioxolane (DX) in admixture with a second solvent which is an acyclic (non cyclic) ether based solvent. The acyclic (non cyclic) ether based solvent as referenced may be dimethoxyethane (DME), ethyl glyme, diglyme and triglyme, with the preferred being 1,2-dimethoxyethane (DME). As given in the example the dioxolane and 1,2-dimethoxyethane (DME) are present in the electrolyte in substantial amount, i.e., 50 vol % 1,3-dioxolane (DX) and 40 vol % dimethoxyethane (DME) or 25 vol % 1,3-dioxolane (DX) and 75 vol. % dimethoxyethane (DME) (col. 7, lines 47-54). A specific lithium salt ionizable in such solvent mixture(s), as given in the example, is lithium trifluoromethane sulfonate, LiCF3SO3. Another lithium salt, namely lithium bistrifluoromethylsulfonyl imide, Li(CF3SO2)2N is also mentioned at col. 7, line 18-19. The reference teaches that a third solvent may optionally be added selected from 3,5-dimethylisoxazole (DMI), 3-methyl-2-oxazolidone, propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), tetrahydrofuran (THF), diethyl carbonate (DEC), ethylene glycol sulfite (EGS), dioxane, dimethyl sulfate (DMS), and sulfolane (claim 19), with the preferred being 3,5-dimethylisoxazole.
In U.S. Pat. No. 6,218,054 (Webber) is disclosed an electrolyte solvent system wherein dioxolane-based solvent and dimethoxyethane-based solvent are present in a weight ratio of about 1:3 (1 part by weight dioxolane to 3 parts by weight dimethoxyethane).
In U.S. Pat. No. 6,849,360 B2 (Marple) is disclosed an electrolyte for an Li/FeS2 cell, wherein the electrolyte comprises the salt lithium iodide dissolved in the organic solvent mixture comprising 1,3-dioxolane (DX), 1,2-dimethoxyethane (DME), and small amount of 3,5 dimethylisoxazole (DMI). (col. 6, lines 44-48.) This reference discloses an anode of lithium alloyed with aluminum.
In US 2007/0202409 A1 (Yamakawa) it is stated with reference to the electrolyte solvent for the Li/FeS2 cell at para. 33: “Examples of the organic solvent include propylene carbonate, ethylene carbonate, 1,2-dimethoxy ethane, γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, sulfolane, acetonitrile, dimethyl carbonate, and dipropyl carbonate, and any one of them or two or more of them can be used independently, or in a form of a mixed solvent.” Such statement is misleading, since the art teaches only specific combinations of electrolyte solvents will be workable for the Li/FeS2 cell depending on the particular lithium salt to be dissolved in the solvent. (See, e.g. above U.S. Pat. No. 5,290,414 and U.S. Pat. No. 6,849,360) The reference Yamakawa does not teach which combination of solvents from the above list are to be used with any given lithium salt.
In U.S. 2006/0046152 (Webber) is disclosed an electrolyte system for a lithium cell which may have a cathode comprising FeS2 and FeS therein. The disclosed electrolyte contains lithium iodide salt dissolved in a solvent system comprising a mixture of 1,2-dimethoxypropane and 1,2-dimethoxyethane.
The choice of a particular organic solvent or mixture of different organic solvents for use in conjunction with any one or more lithium salts to produce a suitable electrolyte for the Li/FeS2 cell is challenging. This is not to say that the cell with various combinations of lithium salt and solvent mixtures may not work at all, but it may not work well enough to be practical. The challenge associated with such cells using an electrolyte formed with just any combination of lithium salt and known organic solvent suitable for dissolution and ionization of the salt is that the problems encountered will likely be very substantial, thus making the cell impractical for commercial usage. The history of development of lithium cells in general, whether lithium primary cells, e.g. non rechargeable Li/MnO2 or Li/FeS2 cells or rechargeable lithium or lithium ion cells reveals that just any combination of lithium salt and organic solvent cannot be expected to result in a good cell, that is, exhibiting good, reliable performance. Thus, references which merely provide long lists of possible organic solvents for Li/FeS2 cells do not necessarily teach combinations of solvents or combination of specific lithium salts in specific solvent mixtures, which exhibit particular or unexpected benefit.
An effective electrolyte for the Li/FeS2 cell promotes ionization of the lithium salt in the electrolyte and is sufficiently stable that it does not degrade with time and does not degrade the anode or cathode components. The effective electrolyte comprises a lithium salt dissolved in an organic solvent mixture which provides good ionic mobility of the lithium ions through the electrolyte so that the lithium ions may pass at good transport rate from anode to cathode through the separator.
The iron disulfide is purchased in the form of a powder. It has exposure to atmospheric air and moisture during transit and storage. This results in contaminants, which include mostly acids and Fe containing salts, forming on the surfaces and within the pores of the FeS2 particles. The contaminants include acids and Fe containing salts such as FeS, H2S, H2SO4, H2SO3, FeSO4, FeSO4 nH2O (hydrate). If these contaminants are present in the cathode, they can react directly with electrolyte or cell components to significantly interfere with proper performance of the cell. It has been determined that if the FeS2 particles are heat treated in a nitrogen atmosphere prior to their use in the cathode mixture, the level of contaminants can be reduced. But it has been found that the contaminants can gradually reform and reappear on the FeS2 surfaces when the heat treated particles are subsequently placed in storage with exposure to atmospheric air and moisture. In a cell assembly operation it is not practical to heat treat the FeS2 particles and use the heat treated FeS2 particles immediately in forming the cathode slurry without exposing them to atmospheric air and moisture prior to forming the slurry.
Conventional FeS2 powders, for example Pyrox Red 325 powder from Chemetall GmbH, are conventionally available with pH raising additives therein to offset or retard any buildup in acidity of the powder. Such additives are believed to contain calcium carbonate (CaCO3). Such calcium carbonate is added to the FeS2 powder to retard the formation of acidic impurities within or on the surface of the powder as it is stored in ambient air and exposed to oxygen and moisture. This is regardless of whether the FeS2 is intended for use in cathode mixtures or other applications, for example, as an additive in manufacture of car brakes.
The addition of pH raising additive such as calcium carbonate (CaCO3) or calcium carbonate containing compounds to the FeS2 powder, however, tends to cause agglomeration of the FeS2 particles when the FeS2 powder is stored in ambient air. Such agglomeration of the FeS2 powder can significantly interfere with attainment of the expected level of performance from Li/FeS2 cells. Also, the calcium carbonate or calcium carbonate containing compound additives has the disadvantage that such compounds get carried into the FeS2 cathode mixture. The calcium carbonate acts merely as an insulator within the cathode, that is, it is not electrochemically active and does not render the cathode more conductive. In other words the calcium carbonate takes up a certain amount of volume within the cathode that might otherwise be used for FeS2 active material. If calcium carbonate is admixed with FeS2 to raise pH it may typically comprise up to about 2.5 percent (maximum) by weight of the mixture. However, calcium carbonate is less dense than FeS2. The bulk density ratio of FeS2 to CaCO3 is about 1.66. That is, 2.5 grams of calcium carbonate has the same volume as 4.15 grams FeS2. Thus for every 2.5 grams of calcium carbonate present in the cathode there are 4.15 grams less FeS2 active material that can be included in the cathode.
In U.S. Pat. No. 4,913,988 (Langan) is discussed the use of the combination of additives Li2CO3 and Ca(OH)2 which are mixed into the cathode of a nonaqueous cell. The Li2CO3 may comprise between about 0.5 to about 2 percent, preferably about 1 percent by weight of the cathode and the Ca(OH)2 may comprise between about 0.5 and 6 percent, preferably 1.3 percent by weight of cathode. The nonaqueous cell may have an active metal anode, for example, lithium metal (claim 4). Manganese dioxide is indicated as the preferred cathode active material for the nonaqueous cell. (col. 3, lines 14-15) The reference focuses on the nonaqueous cell having a lithium anode and cathode comprising principally manganese dixoide (Li/MnO2 cell) and adding the Li2CO3 and Ca(OH)2 to the manganese dioxide cathode. However, the reference also mentions that such nonaqueous cells may be employed with other cathode materials, for example, iron sulfide (theoretical discharge capacity of 893 mAh per gram) or copper oxide (theoretical discharge capacity of 674 mAh per gram). In any event the reference is specific that the cell is a nonaqueous cell. The nonaqueous cell employs an electrolyte which is composed of a salt dissolved in appropriate organic (nonaqueous solvent). (col. 1, lines 26-30). The reference teaches that when manganese dioxide is intended to be used as the cathode active material, the water present in the manganese dioxide can be first removed, by heating it in air or inert atmosphere at a temperature of about 350° C. for about 8 hours. (col. 4, lines 4-10) The reference indicates that the Li2CO3 and Ca(OH)2 may be mixed with the manganese dioxide by the “wet process”, that is, in the presence of some water. In that case the mixture is subsequently dried by heating it at a temperature of between about 120° C. and 150° C. until sufficiently dry before the mixture is shaped into the desired cathode configuration. It is clear that the final cell containing the Li2CO3 and Ca(OH)2 cathode additive is a “nonaqueous” cell.
Accordingly, in the present invention it is desired to reduce the rate of buildup of acidic contaminants in the FeS2 powder and thus in effect to extend the storage life of the FeS2 without sacrificing electrochemical cathode capacity. It is desired to reduce the amount of pH raising additive, such as calcium carbonate, that acts merely as an insulator and takes up volume within the cathode, which could otherwise be used for additional FeS2 active material. But at the same time it is desired to add water to the electrolyte for the cell so that the cell is no longer considered a nonaqueous cell.
It is also desired to improve the method of forming the cathode for the Li/FeS2 cell, in particular to reduce the amount of acidic contaminants carried into the cell by the FeS2 powder.
It is desired to produce a primary (nonrechargeable) Li/FeS2 cell having good rate capability that the cell may be used in place of rechargeable batteries to power digital cameras.